1. Field of the Invention
The present invention relates to a technical field in which nondestructive inspection is performed in piping including a pipe by use of a guided wave so as to collectively evaluate, over long distance, a defect such as wastage that is assumed to occur in a material.
2. Description of the Related Art
Piping used in a power plant, a chemical plant, or the like may be subjected to corrosion and erosion on its inner surface due to the influence of liquid or gas flowing through the piping during the long-term operation of the plant. As a result, deterioration of the piping is promoted in some cases and what is worse, a hole may be disadvantageously bored in the piping in the thickness-wise direction thereof. In such a case, fluid inside the piping such as liquid or vapor leaks and thereby normal operation of the plant will not be performed. As a result, the plant is obliged to stop the operation thereof for a long time. For this reason, it is necessary to evaluate the thickness of piping and a state of material using a nondestructive inspection technique, and then to take measures including the replacement or repair of the piping before the leakage occurs.
One of typical examples of the above-described nondestructive inspection technique is a thickness measurement method using an ultrasonic thickness gauge, which is specified in JIS Z 2355. The thickness measurement method using the ultrasonic thickness gauge is a method for measuring the thickness of piping by exciting an elastic wave in a thickness direction of target piping using an ultrasonic probe, and then by receiving the elastic wave reflected from the bottom surface of the piping using the ultrasonic probe. This ultrasonic thickness gauge measures the thickness of piping by use of the ultrasonic wave propagation time based on a received wave and the known sound velocity and is capable of measuring the thickness of piping with a high degree of accuracy. However, an effective inspection range is a level of the piping contact area of a probe. If a piping diameter is large, or if an inspection range becomes wide (for example, piping whose length ranges from several meters to several tens of meters), the number of measuring points to be measured by the ultrasonic thickness gauge will increase. As a result, it disadvantageously takes much time to complete the inspection. In addition, in the case of such piping that it is difficult for an inspector and an inspection apparatus to access the piping (for example, piping covered by a heat insulator, buried piping, and vertical piping which extends to a high place), it will take much time for the preparation of inspection and clearance work after inspection.
To overcome the above-mentioned drawbacks, inspection technology using a guided wave, which can collectively inspect a long-distance wide area, has been introduced. The principles of a guided wave will be briefly described with reference to FIG. 1. A plurality of ultrasonic sensors or magnetostrictive sensors are arrayed in the circumference direction of piping 1. When the sensors are excited, a guided wave mode is generated in which a guided wave propagates through a material in the piping. Because guided waves have characteristics that the energy thereof is not easily attenuated, the wave motion propagates over long distance. In addition, when the exciting time is controlled, a wave motion mode which allows a defect to be easily detected can be transmitted. If an unevenness portion in thickness of piping is present in the propagation direction (for example, if there is a wastage portion), a guided wave propagating through a straight piping scatters a reflected wave towards the upstream side of a transmitted wave. Thus, receiving the scattered reflected wave makes it possible to detect a defect.
The conventional inspection method that uses a guided wave is described in JP-A-2004-301540. According to this method, a guided wave transmission technique is used to measure the depth of wastage of piping, and the inspection result obtained is then displayed as an image for its evaluation.